Process for the production of a casting layer for the manufacture of films and sheets



United States Patent U.S. C]. 1175.3 4 Claims ABSTRACT OF THE DISCLOSUREA process for producing a casting layer for casting films offilm-forming polymers wherein a known superficially hydrolyzed celluloseacetate layer applied to a metal support is treated with a solution inan organic solvent or a mixture of water and a water-miscible organicsolvent of a Werner chromium complex salt to produce a thin coating ofsuch salt on the layer surface, the coating being then heated to about50 C. to firmly bond the coating to the layer surface.

This invention relates to the manufacture of films or foils from afilm-forming solution, and is concerned in particular with the materialemployed for forming the high-gloss-surface, hereinafter called thecasting layer, to which the film-forming solution is applied and fromwhich the film is stripped off.

Films and foils can be prepared by casting a filmforming solution, e.g.a solution of cellulose triacetate or of another film-forming polymer,on a rotating metal drum or on an endless metal belt forming part of afilm casting machine. After evaporation of the solvent or mixture ofsolvents the film formed adheres firmly to the casting surface so thatit is difficult to detach it therefrom without deforming the filmpermanently.

To obviate adherence thereto the metal surface of the drum or endlessbelt is covered with a casting layer. The layer can only be preparedfrom compounds, which on drying obtain a very smooth high-gloss surface.Moreover, these compounds should be insoluble and even resistant toswelling in the organic solvents customarily used in the manufacture ofthe film-forming solution. At the same time the repellency of thecasting layer towards the filmforming composition should be relativelyhigh, so that the films formed can easily be stripped off withoutpermanent deformation. When the compounds used to form the casting layerdo not possess these properties by nature, it is sufiicient that thecasting layer acquires the abovementioned properties by specialtreatments.

In this way the metal drums or endless belts of film casting machinesare usually provided with a casting layer of cellulose acetate. When themetal drum or endless belt has been pretreated as generally known in theart, the cellulose acetate film adheres firmly to the drum or endlessbelt and dries with a high-gloss surface. In order to confer to thecasting layer the desired repellency towards the film-forming solutionthat is to be applied thereto and to reduce its solubility and swellingtendency in organic solvents, the cellulose acetate layer issuperficially hydrolysed with alkalis.

When casting a film-forming solution on the casting layer treated inthis way, the film obtained after evaporation of the solvent or mixtureof solvents is not of even thickness over the entire width of the film.Indeed, as a result of the evaporation of the solvents the film-formingIce mass contracts towards the middle of the film so that finally a filmwith much thinner edges is obtained. The film tension at the moment ofseparation from the casting layer causes a more or less considerablestretching of the film, which stretching is much more important alongthe edges where the film is thinner. This may even result in a tearingof the film. The parts of the film remaining attached to the castinglayer can easily damage the rather vulnerable casting layer so that thereplacement of this layer may be necessary.

It has been found now that casting layers, which do not suffer fromthese disadvantages, can be prepared by applying an extremely thin layerof a Werner chromium complex salt to the hydrolysed cellulose acetatelayer.

According to the invention a process is found for the production of acasting layer, which comprises coating a metal support with a solutionof cellulose acetate, evaporating the solvent, superficially hydrolysingin a known manner the cellulose acetate layer formed, and applyingthereto a solution in a mixture of water and of a watermiscible organicsolvent of a Werner chromium complex salt in which a trivalent chromiumatom is coordinated with a carboxylic acido group, whereby on thesuperficially hydrolysed cellulose acetate surface a coating having athickness of 0.01 to 5, is obtained, and heating the coating to about C.in order to secure a fir-m bond between the Werner chromium complex saltlayer and the hydrolysed cellulose acetate surface.

The .Werner chromium complex salts, hereinafter called chromium complexsalts, useful in the invention include compounds in which a chromiumatom is coordinated with a carboxylic acido group. The carboxylic acidogroup may derive from a monobasic organic acid, such as acrylic acid,methacrylic acid, stearic acid, palmitic acid and lauric acid wherebythe best results are obtained with the long chain carboxylic acids.Carboxylic acido group may also derive from substituted carboxylicacids, such as p-aminobenzoic acid, cyanoacetic acid and mercaptoaceticacid, and from dicarboxylic acids such as maleic acid. Fluorinatedaliphatic carboxylic acids may also be used in forming the chromiumcomplex salts. The chromium complex salts may be prepared by reaction ofthe appropriate organic acid with chromium compounds by well knownmethods, for example as described in U.S. patent specification 2,273,040filed Sept. 26, 1940 by K. Iler and issued Feb. 17, 1942.

In the process of the invention very good results are obtained withchromium complex salts wherein 1 to 5 chromium groups are present forevery acid radical.

As solvents for the chromium complex salts may be used organic solvents,such as methanol, ethanol, isopropanol, acetone, ethylene glycolmonoethyl ether, ethylene glycol monomethyl ether, and water.

Although Water is a good and cheap solvent for the chromium complexsalts, it has some disadvantages to be used alone. Indeed, water ischaracterised by its very high surface tension. When water would be usedas the sole solvent for the chromium complex salt, a uniform wetting ofthe superficially hydrolysed cellulose acetate layer would not bepossible during the casting of the chromium complex salt solution. Whendrying the chromium complex salt layer, there would always be foundareas where all of the chromium complex salt would be pulled away inconsequence of the high surface tension of water and where consequentlythe superficially hydrolysed cellulose acetate layer would remainuncovered. In order to remedy this disadvantage a certain amount of awater-miscible organic solvent is added, e.g., methanol, ethanol,isopro' panol, acetone, ethylene glycol monomethyl ether, and ethyleneglycol monoethyl ether. Good results are gained when at least 10% oforganic solvent is present calculated on the total amount of water andorganic solvent. A mixture comprising approximately 60% of water and 40%of organic solvent is preferred, however. The chromium complex salts arepreferably applied in very low concentrations, even lower than 0.5 g. ofchromium complex salt per liter of mixture of water and organic solvent.When solutions of much larger concentrations are applied to thesuperficially hydrolysed cellulose acetate layer, the high gloss surfaceof the latter disappears, and the coating of chromium complex salt,which remains after evaporation of the solvent or solvents, is not ofeven thickness over the entire width of the casting layer. Consequently,a very diluted solution of chromium complex salt is applied as anextremely thin layer to the superficially hydrolysed cellulose acetatesurface. The drying time of such a thin layer is extremely short so thatno transport of dry substance can occur under the influence of thesurface tension of water, and the formation of casting lines and cratersis avoided.

In this way the high-gloss of the hydrolysed cellulose acetate surfaceand the repellency characteristic of the thin chromium complex saltlayer are combined. Optimal results are reached with layers of chromiumcomplex salt having a thickness ranging between 0.01 and 5p.-

After application of the chromium complex salts to the hydrolysedcellulose acetate surface, the casting layer is heated for a short timeto about 50 C. in order to secure a good bond between the hydrolysedcellulose acetate and the chromium complex salt layer. Heating to thetemperature indicated for the lapse of time needed to dry the layerapplied and eliminate the solvents by evaporation, may sufiice in mostcases. In some instances, however, the heating time may be increased toone hour and even more.

The coating composition of chromium complex salt dissolved in a mixtureof water and organic solvent can be applied to the superficiallyhydrolysed cellulose acetate surface according to any known coatingsystem e.g. roll coating, dip coating, or knife coating. Care must betaken that the amount of solution applied is such that on drying a layerhaving a thickness of 0.01 to 5/L remains.

The amount of chromium complex salt applied per sur face unit of castinglayer depends on the concentration of chromium complex salt in thesolution and on the quantity of solution applied. Moreover, theconcentration is dependent on the coating system. If e.g. an applicatorroll is used, the amount of applied liquid is dependent on the speed ofthe roll and of the rotating drum or endless belt, on the distancebetween them, and on the number of coatings applied. The result is thatthe concentration of chromium complex salt should be adapted to thecasting machine. When using a low rotation speed of the roll and of thecasting machine and a distance of approximately 0.1 mm. between them,the concentration of chromium complex salt may even be lower than 1 g.per 1000 cc. of solvent mixture, and in this case the application can becarried out in two to four operations.

The repellency of the thin layer of chromium complex salt towards thefilm-forming solution is so strong that the film can be stripped with ahigher solvent concentration from the casting machine without formationof stripping lines. This makes possible an increase of the castingspeed.

The starting and the end of the coating procedure are reproduced visiblyas a line on the casting layer. This line cannot be polished away owingto the extreme thinness of the chromium complex salt layer. In order toovercome this defect the coating procedure is started with a puremixture of solvents. In the supply metering tank this pure mixture isreplaced gradually by the chromium complex salt solution. At the end ofthe coating procedure i.e. after having applied the desired number ofcoatings, the same action is repeated but in reversed sense, so thatfinally a pure mixture of solvents is applied again. In doing so it isimpossible to perceive a beginning or an end of the chromium complexsalt layer. The replacement of the pure solvent mixture by chromiumcomplex salt solution of increasing strength is carried out by means ofa pump system. The pump removes pure solvent mixture and adds equalamounts of chromium complex salt solution. When the appropriate solventconcentration is reached the pump system is used to keep the solutionhomogeneous over the entire width of the coating tray. There are alsoprovided means for keeping the level of the solution in the coating trayrigourously constant.

When the chromium complex salt casting layer is especially intended toavoid the tearing of the film edges and when enhancing of the repellencyof the casting layer as a whole is not so much important, it may sufiiceto apply to both sides of the hydrolysed cellulose acetate sur face asmall strip of the chromium complex salt coating solution according tothe invention.

The casting layer of the invention is suited for the manufacture offilms from solutions in organic solvents of film-forming polymericmaterials such as cellulose triace tate, polycarbonates, polyphenyleneoxide and polyesters such as the copolyester of isophthalic acid,terephthalic acid and 2,2-bis(4-hydroxyphenyl)-propane. The solecondition is that the film-forming polymeric material must be soluble inan organic solvent at ordinary temperatures.

The following examples illustrate the present invention.

EXAMPLE 1 A casting layer of cellulose acetate is applied to the endlessbelt of a film casting machine and hydrolysed superfically according toknown techniques.

A mixture of Water and ethanol (60:40) is then applied to the hydrolysedcellulose acetate surface with the aid of a roll coating machine. Thispure mixture is gradually replaced in the supply metering tank by meansof a pump system by a solution in the same mixture of water and ethanol(60:40) of a chromium complex salt wherein a chromium atom isco-ordinated with an acido-group derived from methacrylic acid. Of themethacrylato-chromic chloride obtained 1 g. is dissolved in 1000 cc. ofthe above indicated solvent mixture.

The roll coating machine is regulated such that approximately cc. ofsolution are coated per sq. m. The coated layer has a thickness ofapproximately l,u., is almost immediately dry; and shows no craters orlines.

After having provided the whole endless belt with a layer ofmethacrylatochromic chloride the solution of chromium complex salt isgradually replaced by the pump system with a pure mixture of water andethanol. In doing so, no starting or end line of the applied chromiumcomplex salt layer can be noticed.

The endless belt is then heated for 2 hours with an air-stream at 50 C.to bring about a good adhesion between the chromium complex salt layerand the superficially hydrolysed cellulose acetate layer.

If a cellulose triacetate solution known from the technique is appliedto the thus formed casting layer and the solvents are partiallyevaporated, the film formed can be stripped much more easily withoutbeing torn or deformed.

EXAMPLE v2 The proces of Example 1 is repeated, with the proviso that inthe chromium complex salt a chromium atom is co-ordinated with an acidogroup deriving from p-aminobenzoic acid. 0.5 g. of the chromium complexsalt is dissolved in 1000 cc. of a mixture 20:80 of Water and ethanol.

A casting layer is obtained from which the cellulose triacetate film canbe stripped off without harm or deformation.

EXAMPLE 3 The process of Example 1 is repeated. The chromium complexsalt used contains a chromium atom co-ordinated with an acido groupderiving from mercaptoacetic acid, i.e. mercaptoacetato chromicchloride. Of this complex salt 0.5 g. is dissolved in 1000 cc. of amixture of water and ethanol (60:40).

The endless belt is heated for 20 mintues at about 50 C. with a hot airstream. The casting layer thus formed enables a cellulose triacetatefilm to be stripped ofi much faster and without being torn.

EXAMPLE 4 The process of Example 1 is repeated, but a mixture of waterand ethanol (50:50) is used as the solvent. The chromium complex saltderives from cyanoacrylic acid, viz, a cyanoacrylato chromic chloride.

The coating system is regulated such that a layer having a thickness ofapproximately 2p. is obtained on drying. The layer is heated for 1 hourto 55 C. by means of a hot air current. The casting layer formed enablesthe cellulose triacetate film to be stripped off much faster and withoutbeing torn.

EXAMPLE 5 The process of Example 1 is repeated. The chromium complexsalt derives from stearic acid, viz. stearatochromic chloride of thiscomplex salt is dissolved in 1000 cc. of solvent mixture.

A layer of stearatochromic chloride of approximately 1, thickness isapplied and this layer is heated for 1 hour at 50 C.

This casting layer enables the cellulose triacetate film to be strippedoff much faster and without being torn.

EXAMPLE 6 The process of Example 1 is repeated. The chromium complexsalt derives from palmitic acid. Of this complex salt 1 g. is dissolvedin 1000 cc. of solvent mixture. Cellulose triacetate film can be easilystripped ofi without being torn at the edges.

EXAMPLE 7 The process of Example 6 is repeated but the palmitato chromicchloride is used in a concentration of 0.5 g. per 1000 cc. of mixture ofwater and ethanol (50:50). Curing of the applied layer occurs by heatingat about 60 C. for 30 minutes.

The cellulose triacetate film which is applied on the casting layer soformed can be stripped off much faster without being torn.

EXAMPLE 8 A casting layer of cellulose acetate is applied to the endlessbelt of a film casting machine and hydrolysed superficially according toa known technique.

According to the method described in Example 1, a layer of about Lu. isapplied to the hydrolysed cellulose acetate surface of a chromiumcomplex salt wherein a chromium atom is co-ordinated with an acido groupderiving from stearic acid. Hardening of the chromium complex salt layeris obtained by heating as in Example 1.

Onto this hardened chromium complex salt layer is coated a solution of apolycarbonate of 2,2-bis(4-hydroxyphenyD-propane in methylene chloride(50 g. of polymer per 100 cc. of solvent).

The films obtained can be taken from the machine when not using theabove chromium complex salt layer.

6 EXAMPLE 9 Example 8 is repeated but after the hardening of thechromium complex salt layer another layer is applied thereto from asolution of polyphenylene oxide in chloroform g. of polymer per cc. ofsolvent). The film obtained does not stick to the casting layer and canbe stripped off very easily.

EXAMPLE 10 Example 8 is repeated but after the hardening of the chromiumcomplex salt layer another layer is applied thereto from a solution ofthe copolyester of isophthalic acid and terephthalic acid (75:25) with2,2bis(4-hydroxyphenyl)-propane (25 g. of polymer per 100 cc. ofsolvent). Stripping off of the film obtained requires but a very smallforce.

We claim:

1. Process for the production of a casting layer for casting films offilm-forming polymers, which comprises coating a metal support with asolution of cellulose acetate, evaporating the solvent, superficiallyhydrolyzing the cellulose acetate layer thus formed, apply thereto asolution in an organic solvent or in a mixture of water and of aWater-miscible organic solvent of a Werner chromium complex salt inwhich a trivalent chromium atom is co-ordinated with a carboxylic acidogroup to produce on the superficially hydrolyzed cellulose acetatesurface coating of said chromium complex salt having a thickness of 0.01to 5p, and heating the coating to about 50 C. in order to secure a firmbond between the chromium complex salt layer and the hydrolyzedcellulose acetate surface.

2. Process according to claim 1, wherein the Werner chromium complexsalt is dissolved in an organic solvent taken from the group consistingof methanol, isopropanol, acetone, ethylene glycol monoethyl ether andethylene glycol monomethyl ether.

3. Process according to claim 1, wherein the Werner chromium complexsalt is dissolved in a mixture of water and of a water-miscible organicsolvent taken from the group consisting of methanol, ethanol,isopropanol, acetone, ethylene glycol monoethyl ether and ethyleneglycol monomethyl ether, said mixture comprising at least 10% ofwater-miscible organic solvent calculated on the total volume of waterand organic solvent present.

4. Process according to claim 1, wherein 1 g. of chromium complex saltis present per 1000 cc. of solvent or solvent mixture.

References Cited UNITED STATES PATENTS 1,529,712 3/1925 Pfiifner 1l7-6O2,726,164 2/1952 Eichrneier 1175.l 2,772,141 11/1956 Dunlap 1l75.13,039,887 6/1962 Schneider et al. 117-5.3

FOREIGN PATENTS 386,775 1/ 1933 Great Britain. 797,050 1/ 1958 GreatBritain.

WILLIAM D. MARTIN, Primary Examiner RAYMOND M. SPEER, Assistant ExaminerUS. Cl. X.R.

